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CKOOI TUITON CENTRE F4 BIOLOGY

Chapter 5 Cell Division

5.1 mitosis

1. cell division involves mitosis and cytokinesis.

2. Mitosis involves the division of one cell into two new cells that are genetically

identical to their parent cell.

3.cytokinesis is the division of cytoplasm into two.

4.the significance of mitosis:

(a) increase the number of cell during growth and development

(b) replace dead or worn out tissues (exp: malpighian layer of the skin tissue)

(c) some organism regenerate lost part of their bodies ( crab and lizard)

(d) asexual reproduction of some microorganisms such as binary fission of amoeba

and budding of yeast

(e) elongation of shoot and root of meristematic tissues of the plants

(f) repaired injured organs ( donate organs)

(g) the growth of new plants from vegetative reproduction

Have you ever wondered how it is possible for our human body to go through

growth and development? What happens when cells in our body are

damaged? The answer lies in cell division.

New cells replace old and damaged ones, and increase in number and size

that lead to our growth. The cell division that contributes to the

replacement of cells as well as tissue growth and repair is known as mitosis.

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How is this likeness formed?

The DNA existing within the chromosomes of a cell's nucleus can make an

exact copy of itself. This means that all chromosomes within the nucleus

duplicate (or replicate) themselves. That is why when the cytoplasm divides

later, each of the two daughter cells has exact copies of the original

chromosomes and DNA!

Note: During the division, the cell splits the copied chromosomes equally to

make sure that each daughter cell has a full set.

Refer to the following diagram, which depicts the series of stages, known as

the cell cycle, undergone by a cell that is about to divide. Basically, the cell

grows, copies (or duplicate) its chromosomes, and then divides to form two

new and identical cells.

Another type of cell division is known as meiosis. Meiosis involves the

division of a cell into four daughter cells. It takes place only in reproductive

organs (eg: in the testes and ovaries of animals and in the anthers and

ovules of plants).

The purpose of meiosis is to produce gametes or reproductive cells so that

sexual reproduction in organisms can occur.

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Unlike those produced through mitosis, these daughter cells are usually not

genetically identical to their parent cells. The chromosomes in each

daughter cell are half the number found in the parent cell.

While cell division occurs once during mitosis, cell division occurs twice

during meiosis. Meiosis I is followed by Meiosis II.

In Meiosis I, the cell divides into two daughter cells, whose number of

chromosomes is halved. In Meiosis II, each of the two daughter cells divides

into another two daughter cells, resulting in four haploid daughter cells

(with the same halved number of the original chromosomes).

Shown in the following diagram are the basic differences between mitosis

and meiosis.

In biology, meiosis (mass) is a process of reductional division in which

the number of chromosomes per cell is halved. In animals, meiosis always

results in the formation of gametes, while in other organisms it can give rise

to spores. As with mitosis, before meiosis begins, the DNA in the original

cell is replicated during S-phase of the cell cycle. Two cell divisions

separate the replicated chromosomes into four haploid gametes or spores.

Meiosis is essential for sexual reproduction and therefore occurs in all

eukaryotes (including single-celled organisms) that reproduce sexually. A

few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry

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out meiosis and have acquired the ability to reproduce by parthenogenesis.

Meiosis does not occur in archaea or bacteria, which reproduce via asexual

processes such as binary fission.

During meiosis, the genome of a diploid germ cell, which is composed of

long segments of DNA packaged into chromosomes, undergoes DNA

replication followed by two rounds of division, resulting in four haploid cells.

Each of these cells contain one complete set of chromosomes, or half of the

genetic content of the original cell. If meiosis produces gametes, these cells

must fuse during fertilization to create a new diploid cell, or zygote before

any new growth can occur.

Thus, the division mechanism of meiosis is a reciprocal process to the

joining of two genomes that occurs at fertilization. Because the

chromosomes of each parent undergo genetic recombination during meiosis,

each gamete, and thus each zygote, will have a unique genetic blueprint

encoded in its DNA. Together, meiosis and fertilization constitute sexuality

in the eukaryotes, and generate genetically distinct individuals in

populations.

In all plants, and in many protists, meiosis results in the formation of

haploid cells that can divide vegetatively without undergoing fertilization,

referred to as spores. In these groups, gametes are produced by mitosis.

Meiosis uses many of the same biochemical mechanisms employed during

mitosis to accomplish the redistribution of chromosomes. There are several

features unique to meiosis, most importantly the pairing and genetic

recombination between homologous chromosomes. Meiosis comes from the

root -meio, meaning less.

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Events involving meiosis, showing

chromosomal crossover

Because meiosis is a "one-way" process, it cannot be said to engage in a cell

cycle as mitosis does. However, the preparatory steps that lead up to

meiosis are identical in pattern and name to the interphase of the mitotic

cell cycle.

Interphase is divided into three phases:

Growth 1 (G1) phase: This is a very active period, where the cellsynthesizes its vast array of proteins, including the enzymes and

structural proteins it will need for growth. In G1 stage each of the

chromosomes consists of a single (very long) molecule of DNA. Inhumans, at this point cells are 46 chromosomes, 2N, identical to

somatic cells.

Synthesis (S) phase: The genetic material is replicated: each of itschromosomes duplicates, producing 46 chromosomes each made up of

two sister chromatids. The cell is still considered diploid because it

still contains the same number of centromeres. The identical sister

chromatids have not yet condensed into the densely packagedchromosomes visible with the light microscope. This will take place

during prophase I in meiosis.

Growth 2 (G2) phase: G2 phase is absent in MeiosisInterphase is followed by meiosis I and then meiosis II. Meiosis I consists of

separating the pairs of homologous chromosome, each made up of two sister

chromatids, into two cells. One entire haploid content of chromosomes is

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contained in each of the resulting daughter cells; the first meiotic division

therefore reduces the ploidy of the original cell by a factor of 2.

Meiosis II consists of decoupling each chromosome's sister strands

(chromatids), and segregating the individual chromatids into haploid

daughter cells. The two cells resulting from meiosis I divide during meiosis II,

creating 4 haploid daughter cells. Meiosis I and II are each divided into

prophase, metaphase, anaphase, and telophase stages, similar in purpose to

their analogous subphases in the mitotic cell cycle. Therefore, meiosis

includes the stages of meiosis I (prophase I, metaphase I, anaphase I,

telophase I), and meiosis II (prophase II, metaphase II, anaphase II,

telophase II).

Meiosis generates genetic diversity in two ways: (1) independent alignment

and subsequent separation of homologous chromosome pairs during the first

meiotic division allows a random and independent selection of each

chromosome segregates into each gamete; and (2) physical exchange of

homologous chromosomal regions by recombination during prophase I results

in new genetic combinations within chromosomes.

A diagram of the meiotic

phases.

Meiosis I

Meiosis I separates homologous chromosomes, producing two haploid cells

(23 chromosomes, N in humans), so meiosis I is referred to as a reductional

division. A regular diploid human cell contains 46 chromosomes and is

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considered 2N because it contains 23 pairs of homologous chromosomes.

However, after meiosis I, although the cell contains 46 chromatids it is only

considered as being N, with 23 chromosomes, because later in anaphase I

the sister chromatids will remain together as the spindle pulls the pair

toward the pole of the new cell. In meiosis II, an equational division similar

to mitosis will occur whereby the sister chromatids are finally split, creating

a total of 4 haploid cells (23 chromosomes, N) per daughter cell from the

first division.

Prophase I

Homologous chromosomes pair (or synapse) and crossing over (or

recombination) occurs - a step unique to meiosis. The paired and replicated

chromosomes are called bivalents or tetrads, which have two chromosomes

and four chromatids, with one chromosome coming from each parent. At

this stage, non-sister chromatids may cross-over at points called chiasmata

(plural; singular chiasma).

Leptotene

The first stage of prophase I is the leptotene stage, also known as

leptonema, from Greek words meaning "thin threads". During this stage,

individual chromosomes begin to condense into long strands within the

nucleus. However the two sister chromatids are still so tightly bound that

they are indistinguishable from one another.

Zygotene

The zygotene stage, also known as zygonema, from Greek words meaning"paired threads", occurs as the chromosomes approximately line up with

each other into homologous chromosomes. This is called the bouquet stage

because of the way the telomeres cluster at one end of the nucleus.

Pachytene

Thepachytene stage, also known aspachynema, from Greek words meaning

"thick threads", contains the following chromosomal crossover. Nonsister

chromatids of homologous chromosomes randomly exchange segments of

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genetic information over regions of homology. (Sex chromosomes, however,

are not wholly identical, and only exchange information over a small region

of homology.) Exchange takes place at sites where recombination nodules

(the aforementioned chiasmata) have formed. The exchange of information

between the non-sister chromatids results in a recombination of information;

each chromosome has the complete set of information it had before, and

there are no gaps formed as a result of the process. Because the

chromosomes cannot be distinguished in the synaptonemal complex, the

actual act of crossing over is not perceivable through the microscope.

Diplotene

During the diplotene stage, also known as diplonema, from Greek words

meaning "two threads", the synaptonemal complex degrades and

homologous chromosomes separate from one another a little. The

chromosomes themselves uncoil a bit, allowing some transcription of DNA.

However, the homologous chromosomes of each bivalent remain tightly

bound at chiasmata, the regions where crossing-over occurred. The

chiasmata remain on the chromosomes until they are severed in Anaphase I.

In human fetal oogenesis all developing oocytes develop to this stage and

stop before birth. This suspended state is referred to as the dictyotene

stage and remains so until puberty. In males, only

spermatogonia(Spermatogenesis) exist until meiosis begins at puberty.

Diakinesis

Chromosomes condense further during the diakinesis stage, from Greek

words meaning "moving through". This is the first point in meiosis where the

four parts of the tetrads are actually visible. Sites of crossing over entangle

together, effectively overlapping, making chiasmata clearly visible. Other

than this observation, the rest of the stage closely resembles prometaphase

of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into

vesicles, and the meiotic spindle begins to form.

Synchronous processes

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During these stages, two centrosomes, containing a pair of centrioles in

animal cells, migrate to the two poles of the cell. These centrosomes, which

were duplicated during S-phase, function as microtubule organizing centers

nucleating microtubules, which are essentially cellular ropes and poles. The

microtubules invade the nuclear region after the nuclear envelope

disintegrates, attaching to the chromosomes at the kinetochore. The

kinetochore functions as a motor, pulling the chromosome along the

attached microtubule toward the originating centriole, like a train on a

track. There are four kinetochores on each tetrad, but the pair of

kinetochores on each sister chromatid fuses and functions as a unit during

meiosis I.

Microtubules that attach to the kinetochores are known as kinetochore

microtubules. Other microtubules will interact with microtubules from the

opposite centriole: these are called nonkinetochore microtubules or polar

microtubules. A third type of microtubules, the aster microtubules, radiates

from the centrosome into the cytoplasm or contacts components of the

membrane skeleton.

Metaphase I

Homologous pairs move together along the metaphase plate: As kinetochore

microtubules from both centrioles attach to their respective kinetochores,

the homologous chromosomes align along an equatorial plane that bisects

the spindle, due to continuous counterbalancing forces exerted on the

bivalents by the microtubules emanating from the two kinetochores of

homologous chromosomes. The physical basis of the independent assortment

of chromosomes is the random orientation of each bivalent along the

metaphase plate, with respect to the orientation of the other bivalents

along the same equatorial line.

Anaphase I

Kinetochore microtubules shorten, severing the recombination nodules and

pulling homologous chromosomes apart. Since each chromosome has only

one functional unit of a pair of kinetochores, whole chromosomes are pulled

toward opposing poles, forming two haploid sets. Each chromosome still

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contains a pair of sister chromatids. Nonkinetochore microtubules lengthen,

pushing the centrioles farther apart. The cell elongates in preparation for

division down the center.

Telophase I

The last meiotic division effectively ends when the chromosomes arrive at

the poles. Each daughter cell now has half the number of chromosomes but

each chromosome consists of a pair of chromatids. The microtubules that

make up the spindle network disappear, and a new nuclear membrane

surrounds each haploid set. The chromosomes uncoil back into chromatin.

Cytokinesis, the pinching of the cell membrane in animal cells or the

formation of the cell wall in plant cells, occurs, completing the creation of

two daughter cells. Sister chromatids remain attached during telophase I.

Cells may enter a period of rest known as interkinesis or interphase II. No

DNA replication occurs during this stage.

Meiosis II

Meiosis II is the second part of the meiotic process. Much of the process issimilar to mitosis. The end result is production of four haploid cells (23

chromosomes, 1N in humans) from the two haploid cells (23 chromosomes,

1N * each of the chromosomes consisting of two sister chromatids) produced

in meiosis I. The four main steps of Meiosis II are: Prophase II, Metaphase II,

Anaphase II, and Telophase II.

Prophase II takes an inversely proportional time compared to telophase I. In

this prophase we see the disappearance of the nucleoli and the nuclearenvelope again as well as the shortening and thickening of the chromatids.

Centrioles move to the polar regions and arrange spindle fibers for the

second meiotic division.

In metaphase II, the centromeres contain two kinetochores that attach to

spindle fibers from the centrosomes (centrioles) at each pole. The new

equatorial metaphase plate is rotated by 90 degrees when compared to

meiosis I, perpendicular to the previous plate.

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This is followed by anaphase II, where the centromeres are cleaved,

allowing microtubules attached to the kinetochores to pull the sister

chromatids apart. The sister chromatids by convention are now called sister

chromosomes as they move toward opposing poles.

The process ends with telophase II, which is similar to telophase I, and is

marked by uncoiling and lengthening of the chromosomes and the

disappearance of the spindle. Nuclear envelopes reform and cleavage or cell

wall formation eventually produces a total of four daughter cells, each with

a haploid set of chromosomes. Meiosis is now complete and ends up with

four new daughter cells.

Meiosis facilitates stable sexual reproduction. Without the halving of ploidy,

or chromosome count, fertilization would result in zygotes that have twice

the number of chromosomes as the zygotes from the previous generation.

Successive generations would have an exponential increase in chromosome

count.

In organisms that are normally diploid, polyploidy, the state of having three

or more sets of chromosomes, results in extreme developmental

abnormalities or lethality. Polyploidy is poorly tolerated in most animal

species. Plants, however, regularly produce fertile, viable polyploids.

Polyploidy has been implicated as an important mechanism in plant

speciation.

Most importantly, recombination and independent assortment of

homologous chromosomes allow for a greater diversity of genotypes in the

population. This produces genetic variation in gametes that promote genetic

and phenotypic variation in a population of offspring.

The normal separation of chromosomes in meiosis I or sister chromatids in

meiosis II is termed disjunction. When the separation is not normal, it is

called nondisjunction. This results in the production of gametes which have

either too many of too few of a particular chromosome, and is a common

mechanism for trisomy or monosomy. Nondisjunction can occur in the

meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.

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This is a cause of several medical conditions in humans (such as):

Down Syndrome - trisomy of chromosome 21 Patau Syndrome - trisomy of chromosome 13 Edward Syndrome - trisomy of chromosome 18 Klinefelter Syndrome - extra X chromosomes in males - ie XXY, XXXY,

XXXXY

Turner Syndrome - lacking of one X chromosome in females - ie XO Triple X syndrome - an extra X chromosome in females XYY Syndrome - an extra Y chromosome in males

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1. The diagram shows a somatic cell during mitosis.

o a) State the mitotic phase in the diagram and give your reason. Metaphase. Chromosomes are arranged at the equatorial plane.

o b) After the mitosis is completed, i) how many daughter cells are produced?

2. ii) how many chromosomes are there in each daughter cell?

4.o c) Give an example of technology currently used which applies the

mitotic process.

Cloning / tissue culture.o d) Name 2 cells which do not undergo mitosis.

i) Nerve cell. ii) Red blood cell.

2. The figure below shows the different stages of cell division in the mammalianovary.

a) Using the letters in the diagram, arrange the stages of cell division in their

correct sequence.

o G ? F ? A? C? E ? B? Db)i) Name the type of cell division illustrated in the figure above.

o

Meiosis.

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ii) With reference to the figure only, state two evidences to support your

answer in [b) i].

o There are 2 cell divisions. 4 daughter cells are formed. bivalents are formed.

c) What biological term is used to describe the cells in stages D and G with

respect to the number of chromosomes in their nucleus?

o D : haploid cell.o G : diploid cell.

d) State one difference in appearance between the chromosomes in stage G

and F and give your reason.

o Chromosomes in stage G appear as one stand but the chromosomes instage F appear as two strands.

Reason: As the chromosomes in G continue to thicken, it isthen seen as two sister chromatids in phase F.

3. The cell life cycle of an organism consists of phases X and Y. Phase Xcomprises of subphases P, Q and R. Phase Y comprises processes U dan V.

a)i) Name process U.

o Mitosis.ii) State the role of process U in living organisms.

o For the growth of organism.o Replace worn-out tissue // for asexual reproduction.

b) Diagrams I, II, III and IV below show the stages in process U.

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i) Arrange the stages of process U in the correct sequence below.

o IV ? II ? III? Iii) Name the stages in process U.

o I

Telophase.II Metaphase.

III Anaphase.

IVProphase.

c) What is phase X?

o Interphase.

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